Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to cooling a computer system. More particularly, the present invention relates to passively and actively cooling a computer system. Still more particularly, the invention relates to independently controlling passive and active cooling in a portable computer in response to processor temperature.
2. Background of the Invention
Computer systems include numerous electrical components that draw electrical current to perform their intended functions. A computer""s microprocessor or central processing unit (xe2x80x9cCPUxe2x80x9d) requires electrical current to perform many functions such as controlling the overall operations of the computer system and performing various numerical calculations. Any electrical device through which electrical current flows produces heat. The amount of heat any one device generates generally is a function of the amount of current flowing through the device.
Typically, each manufacturer designs its products to operate correctly within a predetermined temperature range. If the temperature exceeds the predetermined range (i.e., the device becomes too hot or too cold), the device may not function correctly thereby potentially degrading the overall performance of the computer system. Thus, it is desirable for a computer system generally and its components specifically to operate within a thermally benign environment.
Some computers implement two techniques for cooling the computer""s internal electrical components. One technique, referred to as xe2x80x9cactivexe2x80x9d cooling, uses a fan to blow warm air surrounding one or more xe2x80x9chotxe2x80x9d components through a vent and outside the computer. Thus, active cooling removes the warm air from a computer.
The second cooling technique, called xe2x80x9cpassivexe2x80x9d cooling, slows down the operating speed of a component so that the component will produce less heat. This concept is analogous to a human that travels by foot a given distance. Running the distance at fall speed causes the person to become hotter than merely walking the same distance at a slow pace. To filly appreciate active cooling in a computer, consider that many electrical components, such as integrated circuits (xe2x80x9cICsxe2x80x9d) operate using a xe2x80x9cclockxe2x80x9d signal. A clock signal is a voltage that changes rapidly between a high voltage level (e.g., 3.3 volts) and a low voltage level (e.g., 0 volts) at a predetermined rate. Each transition is called an xe2x80x9cedge.xe2x80x9d The transition of the voltage from low to high and back to low again is called a xe2x80x9ccyclexe2x80x9d and the number of cycles per second is called xe2x80x9cfrequencyxe2x80x9d which is measured in units of Hertz (xe2x80x9cHzxe2x80x9d). For example, a 400 megahertz (MHz) clock signal oscillates at a rate of 400 million cycles per second. Because each cycle includes two voltage transitions (one from low to high and the other from high back to low), a 400 MHz clock signal changes voltage state 800 million times per second.
Most IC""s used in computer systems include transistors. CPUs include thousands or hundreds of thousands of transistor in a single package. Each transistor generally acts as a switch and operates in one of two states-conducting and not conducting. Most computer-related ICs are made from a type of semiconductor technology called Complementary Metal Oxide Semiconductor (xe2x80x9cCMOSxe2x80x9d). Most of the electrical current flowing through transistors, such as CMOS transistors, flows while the transistor changes states from conducting to non-conducting or vice versa. While the transistor is in a particular state, such as conducting or non-conducting, little, if any, current flows through the device.
The transistors in a computer IC change state synchronously with a clock signal. Thus, the transistors in a 400 MHz CPU (i.e. a CPU operating from a 400 MHz clock signal), change state 800 million times per second. Of course, not every transistor in the CPU changes state on every edge of every cycle of the clock signal; some or many transistors may remain in a given state for multiple clock signals. Nevertheless, because current flows through the transistors, which may number in the hundreds of thousands or millions in a typical CPU, on the edges of the clock signal, current flows in spurts 800 millions times per second for a 400 MHz CPU. As the operating speeds of CPUs have increased (i.e., higher frequency clock signals), and likely will continue to increase, the current flowing the CPU has and will continue to increase, all else being equal. Accordingly, heat generated by current flowing in a CPU has become a problem. CPU designers strive to reduce the operating current requirement of their CPUs, but additional measures usually are desirable to reduce the heat generated by the CPU.
The passive cooling technique includes reducing the clock frequency of the CPU. Instead of operating at 400 MHz, the CPU clock will be reduced to lower frequency, such as 300 MHz. With fewer transistor state changes occurring per second, less current flows through the CPU each second and thus, less heat it is generated by the device.
Although the principles of the present invention explained below apply to cooling either a desktop or portable (laptop) computer, the benefit is more pronounced with regard to laptop computers. Laptops operate either from power supplied from a wall socket, alternating current (xe2x80x9cACxe2x80x9d) power, or from a battery, direct current (xe2x80x9cDCxe2x80x9d) power. Typically, the battery is a rechargeable battery that, with a full charge, can power the computer for several hours depending on operating conditions. Because a battery""s charge only operates the computer for a relatively short period of time, it is highly desirable to design laptops to consume as little power as possible.
In contrast to desktop computers, the fans in laptops often can be controlled by the computer""s internal logic thereby permitting the lap top computer to turn off the fan during periods of time in which the fan is not needed to actively cool the computer. Some laptops, in fact, include a temperature sensor to permit monitoring the temperature inside the computer. Further, a temperature sensor is incorporated into some CPUs, such as the Mobile Pentium(copyright) II CPU by Intel(copyright) to permit monitoring of the internal temperature of the CPU. Monitoring the internal temperature of the CPU generally is regarded as beneficial because the CPU is a major contributor to the total heat generated by a computer""s electronics.
Conventional laptop computers typically monitor the computer""s temperature and turn the fan on or off while concurrently adjusting the CPU clock speed. Many conventional laptops monitor the CPU internal temperature and set the state of the fan (i.e., on or off) and at the same time adjust the clock frequency. While generally sufficient to maintain the computer operating in a benign thermal environment, this technique in which the passive and active cooling states are controlled together dependent upon a single temperature reading suffers from at least one problem as explained below.
The problem is most pronounced in laptops which, as noted above, have a limited battery operating life. To conserve battery life, laptops often keep the fan off as much as possible to conserve battery power. That is, whenever not needed for cooling, the computer turns off the fan and when needed, the computer turns on the fan. Of course, the computer also adjusts the CPU clock frequency as it adjusts the fan. The internal temperature of a CPU can vary rapidly as its clock frequency changes. In computers that monitor internal CPU core temperature and that turn the fan on and off and simultaneously adjust CPU clock frequency in response to CPU core temperature, the fan will be turned on and off at an annoying rapid rate because the CPU core temperature can vary rapidly. The annoyance is to the user who can hear the fan turn on and off. On the other hand, if the computer monitors the temperature away from the CPU core, such as on an exterior surface of the CPU package, to avoid the cycling of the fan on and off so rapidly, the computer may unnecessarily slow down the CPU clock to correct a thermal problem that is unrelated to the CPU core.
An additional problem with laptops that control the fan and CPU clock frequency together in response to CPU internal temperature is that air blowing over a CPU generally does not effectively cool a CPU that becomes excessively warm because of internal transistor state changes. In other words, a CPU that becomes hot due to heat generated by its internal core logic is not cooled nearly as well by active cooling from a fan, but rather is cooled much more efficiently by passively adjusting the CPU clock frequency. Thus, often excessive battery power is used to run a fan that has little effect on correcting a thermal problem created by the CPU core.
Accordingly, it is desirable to provide a computer with a thermal control system that solves these problems. Such a computer will reduce or minimize the annoyance caused by a fan turning on and off often in a relatively short period of time. Further, such a computer will have a thermal control mechanism that will effectively maintain the computer sufficiently cool using less power than previously required.
The problems noted above are solved in large part by a computer system having thermal control logic that efficiently cools the computer system. In accordance with one embodiment of the invention, the thermal control logic couples to a CPU module and a fan. The CPU module includes a pair of temperature response elements. One temperature response element located near or on the CPU core logic or die on which the CPU is fabricated. The other temperature response element is located near or on an exterior surface of the CPU module. The thermal control logic monitors the temperature of recorded by each temperature response element and controls the speed of the fan and the frequency of the CPU core clock independently. Preferably, the thermal control logic adjusts the fan speed as a function of the temperature recorded by the temperature element adjacent an exterior surface of the CPU module. The thermal control logic also adjusts the frequency of the CPU clock signal as a function of the temperature recorded by the temperature response element adjacent the CPU core. By disassociating control of the fan speed from the temperature of the CPU core, the annoyance in conventional computer systems caused fans that rapidly turn on and off or change speed rapidly is reduced.
The thermal control logic may be implemented using a keyboard controller that connects to the CPU via a bus, such as a System Management Bus (xe2x80x9cSMBusxe2x80x9d). The CPU includes a temperature sensor that includes the temperature response element located near an exterior surface of the CPU. The other temperature response element is located near the CPU core and connects to the temperature sensor. The temperature sensor includes a pair of registers for storing the temperature values associated with each of the temperature response elements. The keyboard controller polls the CPU for the temperature values recorded by the two temperature response elements. When one of the current temperatures exceeds a predetermined threshold, the keyboard controller generates a system management interrupt (SMI) signal. The CPU core responds to the interrupt by determining which of the temperature response elements triggered the SMI. A pair of control tables preferably stored in main memory coupled to the CPU specify whether the CPU clock frequency should be adjusted in response to the thermal event or whether the fan speed should be adjusted. The CPU core compares the current temperature recorded by the temperature response element that detected the thermal event to the control tables and adjusts either the fan speed or the CPU clock frequency accordingly.
These and other benefits and features will become apparent once the following description is reviewed.